EP1602425B1 - Granulated powder for continuous casting of metals and his manufacturing method - Google Patents

Abstract

Granulate as an insulating or separation means for high temperature applications containing Si dioxide obtained from amorphous Si dioxide powder containing at least 90% SiO 2 by granulation with addition of water and drying. An independent claim is included for a method of obtaining the granulate by mixing SiO 2 with water in solid to water ratio 1:0.25-1:1.5, and granulation and drying of the granulate, where the SiO 2 powder contains at least 90% amorphous SiO 2.

Description

The present invention relates to insulating granules which are suitable as insulating and / or separating agents for high-temperature applications, and to their production.

The current applications of high-temperature-resistant fillers, for example in the iron, foundry and construction industries, use sands and ashes or their mixtures, which are characterized by temperature stability and reaction passivity. These additives are added partly in their dusty form, partly in the form of granules. The powdery substances are considered harmful to health because of the dust effect that occurs and should therefore be successively replaced by granules of comparable functionality.

The granules are to be used in different particle sizes and are produced by spray or pelletizing pelletizing with sometimes considerable technical effort. For the applications in the iron and steel processing industry, a low carbon content is required in addition to the temperature and transport stability, since the carbon content in the additive changes the properties of iron and steel in an undesirable manner. In addition to the mentioned properties, a constant volume of the aggregate over the entire temperature range is desirable in foundry technology, since a change in volume of the fillers and release agents used leads to an unwanted change in the casting geometry. In the construction sector, in contrast to thermal stability, the sound-insulating properties are of outstanding importance.

Out DE 197 28 368 It is known that finely powdered acidic or basic insulating agents are combined in a matrix of gel-forming materials into granules which are suitable as insulating covering agents in the steel foundry. The granules have compared to the powdery insulating agents on a reduced bulk density. Guar gum and hydroxyethyl cellulose are mentioned as gelling materials. A disadvantage of these granules is that from the gel-forming material thus significant amounts of carbon may still be contained, which can lead to contamination of the steel.

There is therefore still a need for separating and insulating agents based on granulated pellets with defined chemical and physical properties in process controllable, reproducible grain sizes, which are characterized by high temperature and material stability and therefore suitable for applications in different industries, where powdery Substances or mixtures thereof or granules with insulating, volume-constant fillers or release agents / additives are used as well as associated production method.

Surprisingly, it has now been found that amorphous silica powder can be processed with the addition of water to give granules which have the desired properties.

The object of the present invention is therefore achieved by isolating granules which are obtainable from an amorphous silica powder by pelleting with the addition of water in a ratio of solids: water of 1: 0.25 to 1: 1.5.

The granules according to the invention offer the advantage of good environmental compatibility in the case of production-controllable grain size or bulk density. In addition to conventional material dosing by hand, an automated task is possible. Experiments have shown that a temperature stability up to 1600 ° C is guaranteed. At the same time, the granules behave inert in terms of reaction and thus offer the certainty that they do not react as inserted filling material with the substances to be processed.

Suitable SiO ₂ suppliers are all silicon powders which contain high proportions of amorphous SiO ₂ or silica. Particularly suitable are microsilica, silica filter dust and mixtures of microsilica or silica filter dust and rice husk ash in different proportions by weight. The addition of further additives for defined bulk density or particle size control is possible. The composition of the starting materials and the mixing ratio of the components involved allows the control of the grain size. At the same time, these parameters also influence the carbon content that is important for the iron and foundry industry.

The silicon dioxide used is preferably silica filter dust, which is obtained without carbon in the silicon or silicon-producing industry and has a consistently high SiO ₂ content of at least 90%. Particularly preferred is a filter dust is used, which is obtained from the flue gas scrubbing the residue combustion in the silicone production. It is irrelevant whether the silicon powder is already in the dried state or in wet chunk form as a filter cake. The silicon dioxide can be used without further drying in the form of the resulting filter cake. The already existing moisture is often enough to cause a direct granulation by stirring. It is expedient to pretreat a caked filter cake by digestion, for example by comminution / milling / sieving, in particular by pressing the puffy SiO ₂ through a perforated screen, so that an open-pored structure important for granulation is obtained.

It has also been shown that a resulting from the dewatering with a chamber filter press filter cake with a moisture content of 60% can be granulated directly without further addition of water, after being previously digested and is in a loosened, open-pore (potting-like) consistency.

For filter slurries with low moisture content of the dried and optionally ground filter cake must be brought by defined addition of water first in the granulation, with a crushing or grinding to achieve the disclosed open-pore structure is also preferred.

In any case, the granulation must be carried out from a defined moisture, which may be based on the used SiO ₂ at a water content of up to 150% based on the total mass. Useful water contents are 25 to 150%; Typically, proportions of 30 to 80%, in particular 30 to 70% are particularly suitable. This slightly empirically determined moisture content must be met quite accurately for a given presented mixture, since too little water does not give rise to granules, only local lumps form in the powder. Further addition of water causes the granules to germinate and grow more and more. This happens until, in a moist granulation equilibrium, the added water combines to form an earth-entrained mass with the germs. By further addition of water, the granulation equilibrium is again shifted to an unfavorable range: the granules lose their structure and, depending on the amount of water, a viscous to light-bodied pasty pulp, which can be moved back into the granulation only by re-addition of solids.

By the stirring time, the granules can be further increased, which, if necessary, reach their final spherical shape after transfer into the granulating. If the quality of the resulting granules in the stirrer is already sufficient, pelletizing in the plate can be dispensed with.

If additional rice husk ash is used as the SiO ₂ supplier, it must be taken into account that the ash does not contain any clumps or husks, since this leads to unwanted, uncontrolled formation of chunks in the production of granules. When adding rice husk ash, a special low-carbon ash with about 5% C content is preferred. The proportion of Reisschalenasche is for cost reasons, preferably up to 30%. This leads to a total carbon content of less than 2%, preferably less than 1%, in the finished granules. Such granules are particularly suitable for use as an additive or insulating agent in the iron and foundry industry.

In the formation of granules, the components used in a targeted manner by pasting with water to form granules. The silicon dioxide and optionally present additives expediently have a particle size which is below, preferably significantly below, the desired granule size. For a granule size of e.g. 0.5 mm, a particle size of the starting materials of not more than 0.1 mm is well suited. A finer grind operated with higher technical effort remains without appreciable effect with this granule size.

In one embodiment, substances which are used as thickening agents in food chemistry are used as additives. For application, these are first prepared in a low-percentage solution in order to influence the size of the granules after already done ateigung of the components to be used with water and an incipient granulation. The addition of dilute thickening agent solution allows the granules to be enlarged as required so that, depending on the area of application, granules of up to several centimeters in diameter can be produced in a targeted manner. Due to the agglomerating property of the additive at the same time an increase in the bulk density is observed, which in turn depends on the used and percentage composition of the SiO ₂ component (s) is. It should be noted that this addition is not required for granule formation.

High-melting oxides or carbonates of the alkali and alkaline earth metals, such as MgO, CaO, CaCO ₃ , Fe ₂ O ₃ , can also be used as additives. In this case, due to the basic or acidic properties of the components in aqueous solution, the pH of the granules produced changes, and the formation of eutectics lowers the melting point of the mixtures in comparison to pure SiO ₂ . To control the pH, it is also possible, if this is not to be achieved by adding an additive, to influence the silicic acid to be used in the aftertreatment by aqueous solutions of acidic or basic precipitation additions so that the granule preparation is already preceded by a filter dust with a defined pH Value takes place. The effects of the precipitants used in filter dust precipitation, which lead to accelerated formation of solids, must be taken into account.

The granules according to the invention are preferably prepared from silicon dioxide powder with the addition of water and optionally additives in a stirrer. As far as e.g. for optimal flow behavior as uniform as possible spherical shape is desired, followed by further processing in a pelletizing.

It is also possible to make the granules by other agglomeration or granulation than pelletizing in a stirrer. Preferably, no or only very small compressive forces should be used in the granulation. Sieve granulators or vertical granulators and fluidized bed granulation are suitable, for example. The well-known Kompaktierungsverfahren are only suitable if a slightly higher bulk density is acceptable.

When producing the granules, initially a SiO ₂ , for example, a silica filter slurry is introduced. This can be immediately pasted while stirring with water; Alternatively, rice husk ash in variable proportions can be added to the filter slurry before the addition of water, which is then mixed with water in the next step as a homogeneous mixture.

When stirring the pasted mixture nuclei are formed after a short time, already containing the structure of the granules to be formed. As the stirring time increases, the diameter of the seeds increases until granules of the desired size are formed. If the added amount of water is too large, the granules lose their consistency again and it forms a pasty porridge, which can be brought back into the necessary for the granulation SiO ₂ water ratio only by adding more Festsoff (filter dust or rice husk ash).

For further control of the granule size, the already formed granules can be treated with the described additives; also by adding Reisschalenasche a granule enlargement is possible. Since the granules already formed during stirring, a subsequent treatment of the same in the granulating is not absolutely necessary, but leads to a regular spherical shape, which is desirable for certain applications.

Regardless of the further treatment, the increase in size can be terminated by spraying with rice husk ash or filter dust, wherein it has been shown that the rice husk ash is particularly well suited for this process. A rice husk ash addition of about 0.5% based on the total mixture is sufficient. It produces dull black granules with a size of 0.5 - 5mm, depending on the stirring time and Reisschalenaschezugabe.

Spraying the granules formed with weakly concentrated thickener liquid as an additive further increases the size, so that the granules can be built up to spheres with a diameter of several centimeters. In contrast to the thickener-free granules, black-gloss granules are produced, which can be powdered again using filler or rice husk ash.

The process of enlarging the resulting granules can be carried out in the same way in the granulating; the granules differ only in the rounder form. An alternative in the granulator is a final spraying of the granules with water, which softens the surface of the granules and leads to a perfect spherical shape due to the rotation in the plate.

It is advantageous to mix the filter dust from the beginning in a predetermined ratio with rice husk ash, thereby the granule size is better controllable. The rice husk ash can be added as required in any weight ratios; In practice, shares of up to 30% ash have led to a good result. At the same time, however, the addition of the rice husk ash increases the bulk density.

Another important aspect for some applications is the pH neutrality of the granules, which are at pH 8. An aggregate should be pH-neutral as changes in pH may have a negative impact on the substance being processed or the forms in which the substance is processed and transported.

In contrast to that DE 197 28 368 Known methods, the present production process without combining the powder used to form a matrix with the aid of a gel-forming material. The use of amorphous SiO ₂ allows direct granulation.

By varying the starting materials, ateigung by defined addition of water and control of the reaction conditions by residence time and inclination / rotational speed of the pelletizing a granulate in a defined grain size of 0.5 - 5 mm size can be produced, which is characterized by thermal and mechanical stability into the high temperature range ,

The granule size can also be influenced by the residence time in the stirrer or later in the pelletizing plate. In both cases, an extension leads to an enlargement of the granules, whereby the evaporation of the water at the granule surface dries them increasingly and the granulation comes to a standstill. An enlargement of the granules is only possible as long as the outside of the granules has a residual moisture, which allows by their pasty surface, an addition of further material. The same applies to the stirring or the granulation: Increasing speed leads to an enlargement of the granules, wherein in particular when stirring an exceeding of a defined number of revolutions of the stirrer depending on container size can have a destructive effect on the granules. By changing the inclination of the granulating a further possibility is given to control the growth of the granules. Due to the steepness, a larger angle of inclination causes the granules to fall at the lower edge of the container due to the greater gravitation and, at the same time, less friction at an increased speed, whereby the coagulation process is intensified. This also leads to an increase in the granule diameter. By downstream screening of the grain size range can be further limited according to the customer's request.

This is important in the filler processing industry, since depending on the application, a certain reproducible particle size is required, which is according to the invention is achieved by varying the production parameters. The grain size is in combination with the bulk density of crucial importance, since the application or introduction of the granules in or on the forms of an independent bleeding for the dosage is necessary.

The previously used powdered materials have the disadvantage of lack of flow properties and are therefore less favorable to handle and dosage. Especially the production of granules with a defined grain size leads to a possibility of customer and application-specific adaptation for the particular application.

The granules according to the invention have a bulk density of 0.2 to 0.7 kg / l, e.g. of 0.5 kg / l and thus meet the demand for a lightweight granules with a low bulk density.

Depending on the customer's wishes, the finished product can be transported and deposited in sacks or BigBags of different sizes, in troughs or silos.

The following examples are intended to illustrate the invention without, however, limiting it to the specific embodiments described. All percentages are by weight unless otherwise specified.

example 1

60 g of microsilica (Elcem, Norway) having the following composition: SiO ₂ : 91.1%, Fe ₂ O ₃ : 2.5%, Al ₂ O ₃ : 0.9%, MgO: 1.1%, K ₂ O. : 1.7%, Na ₂ O: 0.4%, SO ₃ : 0.4%, residual moisture 0.5% and total carbon content. 1.65% was mixed with 30 g of water. The mass was thoroughly mixed in the stirrer, it formed granule nuclei, which increased in size with lasting stirring time. To achieve a smooth surface, the granules were powdered with 5g rice husk ash. The granules had a particle size of 0.5-3 mm and a bulk density of 0.6 kg / l.

Example 2:

48 g of the silica powder from Example 1 were mixed with 12 g of rice husk ash having the following composition
SiO ₂ : 87%, CaO: 1%, Al ₂ O ₃ : 0.2%, Na ₂ O: 0.2%, K ₂ O: 2%, Fe ₂ O ₃ : 0.15%, C _ges : 6.2%, residual moisture 1.1%
and 25 g of water mixed. Subsequently, the mass was stirred in the stirrer, which forms open-pored pellets after a short time, which grow with slowly rotating Rüher. Continued stirring leads to clumps of about 3 mm, which constitute the formation of the granules. Further treatment in the pelletizing plate leads to smooth pellets, which continue to grow. Powdering with SiO ₂ stops the enlargement and leads to a uniform surface and a round structure. The granules had a grain size of 2 - 5 mm and a bulk density of 0.62 kg / l.

Example 3:

160 kg of silica powder with the composition
SiO ₂ : 96.7%, CaO: 1%, Al ₂ O ₃ : 0.2%, Na ₂ O: 0.03%, K ₂ O: 0.01%, Fe ₂ O ₃ : 0.2% , C _tot : 0.75%, residual moisture 1%
were packed with 40 kg of rice husk ash having the following composition:
SiO ₂ : 87%, CaO: 1%, Al ₂ O ₃ : 0.2%, Na ₂ O: 0.2%, K ₂ O: 2%, Fe ₂ O ₃ : 0.15%, C _ges : 6.2%, residual moisture 1.1%
and mixed 138 kg of water in an Eirich mixer. About a doser and a screw conveyor, the conglomerate is fed to a granulating and dried in a belt dryer. By powdering with SiO ₂ or spraying with water, the pellets in the plate can be influenced in size. Screening in the dryer returns dust and large chunks to the Eirich mixer. The pellets had a bulk density of 0.5 kg / l and a softening temperature of 1,600 ° C.

Claims (12)

1. Granulated materials with a bulk weight of 0.2 to 0.7 kg/l as insulating or separating agents for high-temperature applications consisting of a silicon dioxide powder, wherein additives, selected from refractory oxides for changing the pH value and/or melting point of the granulated materials and thickening agents for the adjustment of size may be contained,
   characterized in that the granulated materials are obtainable from an amorphous silicon dioxide powder with a content of at least 90% SiO₂, selected from micro silica, silicon dioxide filter dust, as well as mixtures of micro silica and/or silicon dioxide filter dust with rice husk ashes, by granulating with the addition of water and subsequent drying.
2. Granulated materials according to Claim 1, characterized in that the granulated materials are obtainable by pelletizing.
3. Granulated materials according to Claim 1 or 2, characterized in that silicon dioxide filter dust is contained as silicon dioxide powder.
4. Granulated materials according to Claim 1 or 2, characterized in that mixtures of silicon dioxide filter dust with rice husk ash in different weight percentages are contained as silicon dioxide powder.
5. Granulated materials according to any one of the Claims 1 to 4,
   characterized in that additives for the adjustment of bulk density and, respectively, grain size are contained.
6. Granulated materials according to any one of the Claims 1 to 5,
   characterized in that the overall carbon content is below 2 %.
7. Granulated materials according to any one of the Claims 1 to 6,
   characterized in that the grain size is in the range from 0.5 - 5 mm.
8. Method for manufacturing granulated materials with a bulk weight of 0.2 to 0.7 kg/l as insulating or separating agents for high-temperature applications, wherein

   a) silicon dioxide powder, selected from micro silica, silicon dioxide filter dust, as well as mixtures of micro silica or silicon dioxide filter dust with rice husk ashes, optionally with added thickeners as an additive for adjustment of size and/or added refractory oxides as additive for change of the pH value and/or melting point of the granulated materials, is mixed into a paste with water in the ratio of solids to water from 1 to 0.25 up to 1 to 1.5,

   b) the mass is granulated in a manner known per se, and

   c) the granulated materials are dried,

   characterized in that the silicon dioxide powder contains at least 90% amorphous SiO₂.
9. Method according to Claim 8, characterized in that the mass is pelletized.
10. Method according to Claim 9, characterized in that the grain size of the granulated materials is adjusted to 0.5 - 5 mm via the dwell period and/or the inclination and/or the rotational speed during pelletizing.
11. Method according to any one of Claims 8 to 10, characterized in that the grain size is controlled via the amount of water and/or the composition of the silicon dioxide powder.
12. Method according to any one of the Claims 8 to 11, characterized in that the bulk weight is controlled via the amount of water and/or the composition of the silicon dioxide powder.